1. Field of the Invention
The present invention relates to a carrier used for a two-component developer in an image-forming apparatus such as a digital copying machine and a printer, and more particularly concerns a binder carrier with magnetic particles dispersed in a binder resin.
2. Description of the Prior Art
In copying machines or printers of the electrophotographic system, a two-component developing method has been put into practice in which a two-component developer containing toner and a magnetic carrier is used in developing an electrostatic latent image formed on an image-supporting member such as a photosensitive member.
With respect to carriers used for the two-component developer, various carriers, such as iron particle carrier, ferrite carrier, resin-coated carrier in which these magnetic particles are coated with resin, and binder carrier in which magnetic fine particles are dispersed in a resin, have been known. However, when the resin-coated carrier is carried onto the developing roller as magnetic brushes, its brush is very hard, failing to provide smooth images and causing disturbances in toner images and scratches to the photosensitive member due to scraping. Another problem with the resin-coated carrier is that the developer has a short life because separation of coated layers occurs as it is used.
The binder carrier has been noted as a carrier which can solve the above problem and which easily achieves a small particle size and has a high volume electric resistivity so that it is less susceptible to charge injection from the developer-supporting member.
However, in the conventional binder carrier, a charge-control agent and a post-processing agent tend to adhere to the carrier surface as it is used, causing degradation in the carrier charging-performance and failing to sufficiently charge the toner; this results in image noise such as fog in an image formed on a sheet.
Another problem with the conventional binder carrier is that because of its low electrification-build-up properties, when a toner supply is made after a copying process consuming a large amount of toner, such as a solid black image, toner charging can not be made quickly in a sufficient manner, with the result that fog occurs.